An LED semiconductor luminescent device comprises a semiconductor light-emitting diode (LED), a nixie tube, a symbol tube, a *-shaped tube and a dot matrix display screen (hereinafter referred to as a “matrix tube”) and the like. In fact, each light-emitting unit in the nixie tube, the symbol tube, the *-shaped tube and the matrix tube is a light-emitting diode.
The performance of an LED is determined by parameters of the LED and some key parameters relevant to an LED are as follows: (1) allowable power dissipation Pm, which refers to the maximum value of the product of the forward DC voltage allowed to be applied on both ends of the LED and the passing-through current. If the value is exceeded, the LED becomes hot and can be damaged. (2) Maximum forward DC current IFm, which refers to the allowable maximum forward DC current. If the value is exceeded, diodes can be damaged. (3) Inverse peak voltage VRm, which refers to the allowable inverse peak voltage. If the value is exceeded, the light-emitting diode can be broken down and damaged. (4) Operating temperature topm, which refers to the range of the environment temperature for normal operation of light-emitting diodes, and if the temperature of the operating environment is beyond the temperature range, the light-emitting diodes cannot operate properly and the efficiency is greatly reduced. (5) Forward operating current If, which refers to the forward current value when the light-emitting diode emits light normally. In actual use, the IF should be selected below 0.6·IFm as required. (6) Forward operating voltage VF, and the operating voltage given in the parameter list is achieved with the given forward current and usually measured when IF=20 mA. The light-emitting diode has the forward operating voltage VF within 1.4˜3V. When the external temperature rises, the VF decreases. (7) V-I characteristics, wherein the relationship between voltage and current of a light-emitting diode is that when the forward voltage is just below a given value (called a threshold value), the current becomes extremely low and the LED cannot be lighted up. When the voltage exceeds the value, the forward current rises rapidly and progressively along with the voltage and the LED is lighted up.
Currently, at home and abroad, the LED is activated by means of constant voltage or constant current. No matter under numerical control or simulation control, when a constant-current and constant-voltage LED actuating power supply is equipped to actuate the LED lamp unit, the constant-voltage or constant-current control technology is adopted during operation no matter how great the VF value of the LED lamp unit is changed, that is, the output voltage and current coincide with the rated values of the lamp unit; however, the VF-values of LED lamp units are different from those of different manufacturers, especially the VF-value consistency is much poorer for the LED lamp units from those of the factories with a less advanced packaging technology. And after lamp beads completely different in the VF-value consistency constitute a lamp unit, the VF-values quickly drift and decrease under the impact of the temperature, but the existing LED power supplies all have constant voltage or constant current, so it is completely impossible to adjust the output current and voltage according to the serious drift of the VF-value when the LED lamp unit is exposed to a high temperature, thus forcing the LED lamp unit to operate under the condition of low VF voltage and constant current. As a result, the PN-junction temperature of the LED continues to rise to speed up light depreciation, aging and color temperature deviation of the LED lamp unit.
For example, in time of constant-current actuation, when the output voltage and current of an LED actuating power supply is 36V and 2.4 A respectively, the output power should be 86 W; when the temperature rises and the VF-value deceases, the output voltage has tiny changes: 30V, and the output power is 72 W. The LED lamp unit's temperature continues rising to cause continuous VF-value falling due to the 2.4 A constant-current actuation, while the actual luminous flux of the LED lamp unit is greatly reduced due to lowering of the VF-value, thus speeding up light depreciation, flickering and color temperature deviation of the LED lamp beads when a constant current passes through. In time of constant-voltage actuation, when the output voltage and current of the LED actuating power supply are is 36V and 2.4 A respectively, the output power should be 86 W; when the temperature rises and the VF-value decreases, the output voltage is always adjusted to be 36V via the feedback regulation function of the constant-voltage actuator, while the output current rises to 2.6 A instead, and the output power also rises to be 93.6 W. The continuous rising of the PN-junction temperature of the LED lamp unit results in continuous decreasing of the VF-value, the current further rises because of the feedback regulation function of the constant-voltage circuit, with the consequence that the power continuously kept out of limits, which results in burning of the LED lamp unit.